The Chūō Main Line(中央本線,Chūō-honsen?), commonly called the Chūō Line, is one of the major trunk railway lines in Japan. It connects Tokyo and Nagoya, although it is the slowest direct railway connection between the two cities; the coastal Tōkaidō Main Line is slightly faster, and the Tōkaidō Shinkansen is the fastest rail link between the cities.

The eastern portion, the Chūō East Line(中央東線,Chūō-tōsen?), is operated by the East Japan Railway Company (JR East), while the western portion, the Chūō West Line(中央西線,Chūō-saisen?), is operated by the Central Japan Railway Company (JR Central). The dividing point between the two companies is Shiojiri Station, where express trains from both operators continue to the Shinonoi Line towards the cities of Matsumoto and Nagano. Compared to the huge urban areas at either end of the Chūō Line, its central portion is very lightly traveled; the Shiojiri-Nakatsugawa corridor is only served by twice-hourly local and hourly limited express trains.

The Chūō Main Line passes through the mountainous center of Honshu. Its highest point (near Fujimi Station) is about 900 meters above sea level and much of the line has a gradient of 25 per mil (2.5% or 1 in 40). Along the Chūō East Line section, peaks of the Akaishi and Kiso as well as Mount Yatsugatake can be seen from trains. The Chūō West Line parallels the old Nakasendō highway (famous for the preserved post towns of Tsumago-juku and Magome-juku) and the steep Kiso Valley.

The section between Tokyo and Mitaka is grade-separated, with no level crossings. Between Ochanomizu and Mitaka, the Chūō Main Line has four tracks; two of them are local tracks (緩行線,kankō-sen?) with platforms at every station; the other two are rapid tracks (快速線,kaisoku-sen?) with some stations without platforms. The local tracks are used by the main line local trains (operated only in early morning and late night) and the Chūō-Sōbu Line local trains, while the rapid tracks carry rapid service and limited express trains. The Tokyo-Mitaka portion is a vital cross-city rail link.

The commuter services on the rapid tracks are collectively called the Chūō Line (Rapid) in comparison with the Chūō Line (Local)(中央線各停,Chūō-sen-kakutei?) or the Chūō-Sōbu Line on the local tracks. The former is usually referred to simply as the Chūō Line and the latter the Sōbu Line. Separate groups of trainsets are used for these two groups of services: cars with an orange belt for the rapid service trains and cars with a yellow belt for the local service trains, with the exception of early morning and late night local service trains which use cars with an orange belt. Signs at stations also use these colors to indicate the services.

The four-track section ends at Mitaka. Construction is underway between Mitaka and Tachikawa to elevate the tracks and eliminate level crossings; this section of the line was notorious for its level crossings which can be shut for upwards of an hour during rush hour. Plans have been proposed to add another two tracks as far as Tachikawa, but were not included in the track elevation, which was completed between 2008-2011, with further modifications in 2012.

This section is also entirely in Tokyo. For legends on train types, see the preceding section.

Most of the rapid service trains from Tokyo terminate at Takao where the line exits the large urban area of Tokyo. The section between Takao and Ōtsuki still carries some commuter trains as well as long distance local trains and Limited Express trains. The Kaiji limited express terminates at Kōfu, the capital of Yamanashi Prefecture, while the Azusa and Super Azusa continue beyond Shiojiri to Matsumoto via the Shinonoi Line.

Prior to the opening of the new route between Okaya and Shiojiri, there was a junction (Higashi-Shiojiri Junction (東塩尻信号場?)) between Ono and Shiojiri stations. It had a reversing layout. The signal station was closed on October 12, 1983.

Fumonji Junction (普門寺信号場,Fumonji Shingōjō?) is a junction between Chino and Kami-Suwa stations in Suwa, Nagano. It entered into use on September 2, 1970.

Sannō Junction (山王信号場,Sannō Shingōjō?) is a junction that diverts freight traffic from the Chūō Main Line to the Tōkaidō Line freight branch between Kanayama and Nagoya stations in Nagoya. It entered into use on October 10, 1962.

New E233 series trains entered service on Tokyo-area commuter services from December 26, 2006. These trains are a development of the E231 series used on other commuter lines in the Tokyo area, and replaced the aging 201 series rolling stock introduced on the line in 1981.

From 2016, new E353 series EMUs are scheduled to be introduced on Azusa and Super Azusa limited express services, replacing the E351 and E257 series trains.[1]

The Kobu Railway (甲武鉄道?) opened the initial section of the Chūō Line from Shinjuku Station to Tachikawa Station in 1889.[2] The company then extended the line both westward and eastward (towards Tokyo) until it was nationalised in 1906. The Japanese Government Railways (JGR) then continued to extend the line, reaching Shiojiri the same year, and Tokyo (at Shōheibashi Station (昌平橋駅?)) in 1908.[citation needed] The JGR also built the line from Nagoya, the first section opening in 1900, with the lines connecting in 1911. The Table below gives the section opening dates.

In 1904, the section between Iidamachi Station (formerly located between Suidōbashi Station and Iidabashi Station) and Nakano Station was the first urban electric railway in Japan using 600 V DC. Electrification was extended in 1919 and 1922, was increased to 1,200 V DC when extended to Tokyo in 1927, boosted again to 1,500 V DC in 1929, and reached Kofu in 1931. Electrification from the Nagano end was commissioned in sections from 1966, and the entire line was electrified by 1973.[citation needed]

The initial section double-tracked was from Shinjuku to Nakano in 1906, extending to Asakawa by 1939. Double-tracking from Nagano commenced in 1962, with 85% of the line double-tracked when work ceased in 1983.[citation needed]

CTC signalling was commissioned between Shiojiri and Nakatsu between 1973 and 1974, and extended to Nagoya in 1992.[citation needed]

Mitaka Station: A 3 km line to a Nakajima Aircraft factory opened in 1942, and was out of service in 1945.[citation needed] In 1950, the factory site was used to build a sports stadium. The line from Mitaka to Musashino Stadium (武蔵野競技場前?) opened on 14 April 1951, but was closed from 1 November 1959.[3]

Kokubunji Station: A 7 km line was opened in 1910 to haul gravel from the Tamagawa. It closed in 1914 due to flood damage, but was reopened in 1916 after being rebuilt by the Japanese Army.[citation needed] On 26 May 1920, the line was absorbed into JNR, but operations were suspended from 1 December 1921.[3] A 6 km[citation needed] extension to the Tokyo Racecourse opened on 1934.[3] Services on the line were suspended from 1 October 1944, resuming from 24 April 1947.[3] On 1 April 1973, the line to Tokyo Racecourse closed, and the freight line was absorbed into the Musashino Line.[3]

Kofu Station: The Yamanashi horse-drawn tramway opened its first 660 mm (2 ft 2 in) gauge section in 1898, and by 1904 had opened two lines (to Katsunuma and Fujikawa) totaling 34 km. In 1930, the Katsunuma Line was closed, and the other line was closed beyond Kai-Aoyagi, 20 km from Kofu. The company renamed itself the Yamanashi Electric Railway, regauged (to 1,067 mm) and electrified the line at 600 V DC, and operated it until 1962.

Sakashita Station: The 11 km 762 mm (2 ft 6 in) gauge Sakagawa Line was opened to Maruno by the Hisaka River Railway in 1926. A passenger service was operated 8 km to Okuya. The Forest Service opened a 9 km line connecting at Maruno the same year, and a 2 km branch from Okuya that operated from 1933 until 1958. In 1944, the Forest Service took over the Sakagawa line, operating it until 1961, when the entire 20 km line closed.[citation needed]

Nakatsugawa Station: The Kitaena Railway operated the 23 km Enaden Line to Tsukechi, electrified at 600 V DC, from 1924 until 1978. At Tsukechi, it transshipped timber from a 762 mm (2 ft 6 in) gauge forest railway with an 18 km "main line" and a 14 km and two 5 km branch lines operated from 1932 until 1959.

Ena Station: The Iwamura Electric Railway operated a 13 km line electrified at 600 V DC to its namesake town between 1906 and 1935. A 4 km line to the site of Oi dam was opened in 1922 to transport construction materials. Upon the dam's completion, the line was sold to the Kita-Ena Railway. but it closed in 1934.

Tokishi Station: The Ogawa Railway opened a 10 km line to its namesake town between 1922 and 1924. The line was electrified at 1,500 V DC in 1950, and closed as a result of flood damage in 1972.

Tajima Station: The Kasahara Railway opened a 5 km line to its namesake town in 1928. Passenger services ceased in 1971, and the line closed in 1978.

Chino Station: The Saku Railway, which had built the line from Komoro on the Shinetsu Line to Koumi, proposed to build a line from Tanaka on the Shinetsu Line to this station. The company was nationalised before construction started, and JGR connected the Koumi line to the Chuo Main Line in 1935, making this proposal redundant.